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Design, Synthesis, and Application of NNN Pincer Ligands Possessing a Remote Hydroxyl Group for Ruthenium-Catalyzed Transfer Hydrogenation of Ketones Zhengqiang Cao, Hong Qiao, and Fanlong Zeng*

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Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, National Demonstration Center for Experimental Chemistry Education, College of Chemistry & Materials Science, Northwest University, 1 Xuefu Road, Xi’an, Shaanxi 710127, P. R. China S Supporting Information *

ABSTRACT: A new family of pyridyl-based NNN pincer ligands bearing a remote pendent OH group were developed. Considerable acceleration effects on the activity of Ru-catalyzed transfer hydrogenation of ketones were imparted by the pendent OH group, and importantly, introducing a CH2OH group to the 4′-position of the pyrazolyl moiety is an appropriate choice. The results present a general strategy for exploring bifunctional ligands to construct effective catalysts.

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mechanism of the “NH effect” has not been well understood until recently. The latter studies revealed Noyori’s catalyst requires an alkali metal cocatalyst, and during catalysis, the alkali metal cation coordinates to the deprotonated amino moiety to form an ion pair.18 Very recently, Szymczak and coworkers demonstrated a 2-hydroxypyridyl fragment is a suitable candidate to construct bifunctional catalysts, and also unveiled that the deprotonated hydroxyl groups serve to orient the ketone substrate through ion pairing with alkali metals.19 Mechanistically inspired by advances in metal−ligand cooperation, we envisage metal−ligand cooperation could be achieved by a remote pendent hydroxyl group installed on a proper position of the ligand framework. Herein, we disclose a new family of unsymmetrical pyridyl-based NNN pincer ligands containing a remote pendent hydroxyl group which facilitates cooperative reduction of ketones.

INTRODUCTION Exploration of highly effective catalysts is of major interest to the synthetic community. Ligands usually play a key role in catalytic systems via elaborately modulating electronic, steric, and geometric properties of the catalysts. Traditionally, ligands solely serve to effect the environment around the metal centers, whereas they do not take part in the activation of substrates or the formation of products. The pioneering and seminal work on hydrogenation by Shvo1 and Noyori2 disclosed a new concept for ligand design which established a division of powerful homogeneous catalysis known as metal− ligand bifunctional catalysis.3 Since then, the bifunctional catalysts based on reversible deprotonation/protonation of XH (X = N, O, C) moiety(ies) of the ligands have been well exploited. A number of bifunctional transition-metal catalysts supported by polydentate ligands featuring ancillary NH group(s) were reported by the groups of Zhang,4 Morris,5 Kuriyama,6 Gusev, 7 Pidko, 8 Beller,9 Barrta,10 Guan,11 Schneider,12 Yu,13 Khaskin,14 and Kayaki15 which were successfully applied to hydrogenation, dehydrogenation, and related reactions. Recently, Milstein’s group disclosed a new type of metal−ligand cooperation based on the reversible dearomatization/aromatization of heteroaromatic ligand backbones invoked by deprotonation/protonation of P−CH2− heteroaryl functionalities.16 Similarly, Kempe et al. developed a family of PNP pincer ligands employing P−NH−triazinyl fragments for revisible proton donation/acceptance.17 Despite that numerous active catalysts were developed by taking advantage of the “NH effect” disclosed by Noyori,2 the © XXXX American Chemical Society

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RESULTS AND DISCUSSION The reaction of 2,6-difluoropyridine with 3,5-dimethyl-1Hpyrazole at room temperature selectively produced singly substituted pyridine 3 (73%), which further reacted with electron deficient ethyl 1H-pyrazole-4-carboxylate or ethyl 1Hpyrazole-3-carboxylate at elevated temperature, furnishing doubly substituted pyridine 4 or 5 in good yield. The anticipant ligands 6 and 7 were accessed by reduction of the ester group utilizing LiAlH4 as reductant in THF. The hydroxyl Received: October 29, 2018

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DOI: 10.1021/acs.organomet.8b00791 Organometallics XXXX, XXX, XXX−XXX

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Scheme 3. Synthesis of Ru(II) Pincer Complexes 14−18a

group in 6 was methylated in 94% yield by iodomethane in the presence of NaH (Scheme 1). Scheme 1. Synthesis of Ligands 6−8a

a

Conditions: (i) NaH, DMF, rt, 73%; (ii) ethyl 1H-pyrazole-4carboxylate or ethyl 1H-pyrazole-3-carboxylate, NaH, DMF, 110 °C, 6 h, 81% (4), 88% (5); (iii) LiAlH4, THF, −10 °C to rt, 2 h, 85% (6), 75% (7); (iv) NaH, CH3I, THF, 0 °C to rt, 2.5 h, 94%.

The condensation of pyridyl hydrazine 9 and enaminone 10 directly delivered pyridine 11 in 95% yield. CrO3-catalyzed oxidation of the CH2 group of 11 by tBuOOH in refluxed dichloromethane afforded ketone intermediate 12 in excellent yields, which was then reduced to targeted racemic alcohol 13 by NaBH4 in refluxed MeOH in excellent yield (Scheme 2). Scheme 2. Synthesis of Ligands 11 and 13a

Conditions: (i) AcOH, 117 °C, 4 h, 95%; (ii) CrO3, tBuOOH, CH2Cl2, refluxed, 12 h, 95%; (iii) NaBH4, CH3OH, 85 °C, 3 h, 83%.

a

The reaction of pincer-type ligands with RuCl2(PPh3)3 has been overwhelmingly employed to approach their Ru(II) pincer complexes, which therefore is our first attempt to prepare the targeted ruthenium complexes. Gratifyingly, reacting RuCl2(PPh3)3 (1.0 equiv) with 6−8, 11, and 13 in refluxed toluene under a nitrogen atmosphere successfully afforded the corresponding complexes 14−18 in 79%, 69%, 92%, 83%, and 69%, respectively (Scheme 3). It is worth mentioning that complex 18 is a diastereomeric mixture with a diastereomer ratio of 44:42. The proton NMR of complex 14 revealed a triplet peak at 5.50 ppm for the OH group and a multiplet peak at 4.38 ppm for the CH2 group, whereas the proton NMR of complex 15 showed a triplet peak at 5.64 ppm for the OH group and two dd peaks at 5.14 and 4.35 ppm (J1 = 5.4 Hz and J2 = 14.1 Hz) for the CH2 group. The significant upfield shift of one of the methylene protons in complex 15 in comparison with complex 14 would be ascribed to shielding by the neighboring PPh3 ligand. The single crystal X-ray diffraction analysis of complexes 14−18 discloses they adopt similar structures with distorted ruthenium octahedral geometries (selected bond lengths (Å) and angles (deg) for

complexes 14−18; see the Supporting Information, Table S1). The pincer-type NNN ligand occupies the three meridional positions with the three N-heterocyclic rings in a quasi-planar arrangement. One PPh3 ligand and one Cl ligand are trans to each other on the two sides of the NNN ligand quasi-plane. One chloride ligand is positioned trans to the pyridyl nitrogen atom. It is worthy to note that complex 14 features the shortest Ru1−N1 bond distance, the longest Ru1−N3 and Ru1−N4 bond distances, and the biggest N3−Ru1−N4 bond angle, demonstrating that the bonding pocket of ligand 6 is wider than those of ligands 7, 8, 11, and 13. Additionally, intermolecular hydrogen bonding occurs between the OH group and one of the two Cl ligands in the solid state of complexes 14 and 18 (selected bond lengths (Å) and angles (deg) for hydrogen bonds involving the OH group in complexes 14 and 18; see the Supporting Information, Table S2).

a

Conditions: RuCl2(PPh3)3 (1.0 equiv), toluene, 110 °C, 6 h, under N2, 79% (14), 69% (15), 92% (16), 83% (17), 69% (18). bThe yield (69%) of 18 is for the diastereomeric mixture, and the crystal structure of 18 represents only one of the two diastereomers.

B

DOI: 10.1021/acs.organomet.8b00791 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Transfer hydrogenation (TH) of acetophenone in refluxing 2-propanol was selected as model reaction to evaluate the catalytic activity of complexes 14−18. Employing 0.1 M ketone solution, 0.5 mol % of the complex as precatalyst, and 10 mol % of iPrOK as the base, the complexes with a pendent hydroxyl group, i.e., 14, 15, and 18, demonstrated much higher catalytic activity than the complexes without a pendent hydroxyl group, e.g., 16 and 17 (Figure 1). It is noteworthy

Figure 2. Screening of the bases for TH of acetophenone. All reactions were carried out with 2.0 mmol of acetophenone, acetophenone/cat. = 400:1, 20 mL of iPrOH, under N2, 82 °C. Yield determined by GC.

Figure 1. Screening of the catalysts for TH of acetophenone. All reactions were carried out with 2.0 mmol of acetophenone, acetophenone/iPrOK/cat. = 200:20:1, 20 mL of iPrOH, under N2, 82 °C. Yield determined by GC.

that the location of the OH group significantly affects the catalytic activity of complexes, and introducing a CH2OH group to the 4′-position of the pyrazolyl moiety has proven to be a suitable choice. Complex 14 showed the highest activity, reaching 98% conversion of acetophenone within 30 s with the final TOF up to 23 760 h−1. Under the same conditions, complex 15 and diastereomeric complex 18 offered lower activity, achieving 74% and 53% conversion in 30 s, respectively. Methylation of the hydroxyl group in complex 14, providing complex 16, considerably hampered the catalytic efficiency, only giving 27% conversion in 30 s. Surprisingly, complex 17 with an acidic CH2 group at the 4′-position of pyrazolyl displayed very low catalytic activity, which may be because the acidity of the CH2 group is too weak compared to the base employed. We next chose the most active catalyst complex 14 to optimize other reaction parameters. Lowering the loading of complex 14 to 0.2 mol % slightly reduces the reaction efficiency, providing 98% conversion in 2 min. Further lowering the loading of 14 to 0.1 mol % clearly impeded the reaction efficiency, only furnishing 55% conversion in 2 min. The performance of complex 14 was significantly affected by the employed alkali metal cations and corresponding anions. The results outlined in Figure 2 indicate that the reduction rate is modulated by the identity of the metal cation in the order Li < Na < K, which implies the alkali metal cation is involved during catalysis, and might coordinate to the deprotonated OH group to form an ion pair as described by Szymczak’s group.19 The results also indicate that decreasing the basicity of the employed anion attenuates the catalysis, and anion iPrO− is the optimal choice for this system. Excessively decreasing or increasing the loading of iPrOK is detrimental to the catalysis, and 10.0 equiv of iPrOK to complex 14 is an appropriate amount (Figure 3) With the optimized reaction conditions established, the catalytic activity of complex 14 was further investigated by

Figure 3. Optimization of the amount of iPrOK for TH of acetophenone. All reactions were carried out with 2.0 mmol of acetophenone, acetophenone/cat. = 400:1, 20 mL of iPrOH, under N2, 82 °C. Yield determined by GC.

reducing various ketones (Table 1). Employing 0.2 mol % of complex 14 as the precatalyst and 2 mol % of iPrOK as the base, acetophenone was reduced to 1-phenylethanol in 2 min. The substituents, such as Br, F, I, and MeO, on the position ortho to the acetyl of substituted acetophenones showed an acceleration effect on the TH reduction, reaching 99% conversion in 0.5 min with the highest final TOF up to 59 400 h−1 (Table 1, entries 2−4 and 6). But the methyl group on the same position did not obviously affect the reaction efficiency (Table 1, entry 5). The substituents, i.e., Br, Cl, F, Me, and MeO, on the position para to the acetyl of substituted acetophenones slightly decelerated the rate of reduction, and all the substrates were reduced to the corresponding alcohols within 4 min (Table 1, entries 10−14). The substituents, e.g., Br, Me, and MeO, on the position meta to the acetyl of substituted acetophenones deteriorated the reaction efficiency (Table 1, entries 7−9). 1-(1-Naphthalenyl)ethanone was reduced in 1.0 min with 97% conversion, whereas 1-(2naphthalenyl)ethanone required 4 min to achieve 96% conversion. Complex 14 displayed excellent catalytic activity to 1-indanone, diphenylmethanone, cyclopentanone, cyclohexanone, and 2-octanone, accomplishing over 91% yields in 6 min. This catalytic system is also applicable to heteroaryl ketones, such as 2-acetylfuran and 3-benzoylpyridine (Table 1, entries 22 and 23). C

DOI: 10.1021/acs.organomet.8b00791 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Table 1. TH of Ketones Catalyzed by Complex 14a

A possible mechanism for the present TH reactions is depicted in Scheme 4 with complex 14 as the model catalyst. Scheme 4. Proposed Catalytic Cycle

Initially, 14 possessing an acidic OH group undergoes dehydrohalogenation, anion exchange, and β-H elimination upon exposure to base, generating coordinatively unsaturated ruthenium hydride complex B. Unfortunately, attempts to isolate the 16-electron zwitterionic Ru(II)-H species B failed. There are two possible pathways for the present TH reactions. For path a, a ketone substrate is activated via interaction with K+, which is strongly supported by complex 14 showing higher reactivity than complex 15 and diastereomeric complex 18. In complex 15, the CH2OH group is installed too close to the ruthenium center to properly achieve the orientation of the ketone via ion pair. For diastereomeric complex 18, the isomers with PPh3 and CH2OH cis to each other cannot accomplish the transfer of H− from the metal to the carbonyl as happened in intermediate C, because the PPh3 ligand is cis to the deprotonated OH group. Following the outer-sphere mechanism is also supported by observed cation effects on reduction rate. For path b, a ketone substrate is directly captured by the electrophilic cationic metal center of complex B to give intermediate C′. If following the inner-sphere mechanism, complexes 14 and 18 will exhibit similar reactivity, and also without the observation of cation effects on reduction rate. However, based on these preliminary results, pathway b cannot be excluded and pathway a is more convincing.

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CONCLUSIONS In summary, a new family of pyridyl-based NNN pincer ligands bearing a remote pendent hydroxyl group were developed and their ruthenium complexes exhibited high activity in transfer hydrogenation of ketones with the final TOF up to 59 400 h−1 in the presence of 0.2 mol % catalyst. Importantly, the location

a

Conditions: ketone, 2.0 mmol (0.1 M in 20 mL iPrOH); ketone/ iPrOK/14 = 500:10:1; under N2, 82 °C. bYield determined by GC. D

DOI: 10.1021/acs.organomet.8b00791 Organometallics XXXX, XXX, XXX−XXX

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8.0 Hz, 2H), 7.79 (t, J = 4.0 Hz, 1H), 6.90 (d, J = 4.0 Hz, 1H), 5.94 (s, 1H), 4.41−4.36 (m, 2H), 2.64 (s, 3H), 2.22 (s, 3H), 1.37 (t, J = 8.0 Hz, 3H). 13C{1H} NMR (101 MHz, CDCl3) δ 162.0, 151.9, 150.3, 148.7, 145.9, 141.2, 141.0, 128.2, 113.3, 110.1, 109.8, 108.9, 61.2, 15.2, 14.3, 13.6. HRMS (ESI) m/z calcd for C16H17N5O2 [M + Na]+ 334.1274, found 334.1259. (1-(6-(3,5-Dimethyl-1H-pyrazol-1-yl)pyridin-2-yl)-1H-pyrazol-4-yl)methanol (6). Under a N2 atmosphere, compound 4 (883 mg, 2.8 mmol) was dissolved in THF (10 mL), which was added dropwise to a stirred suspension of LiAlH4 (140 mg, 3.7 mmol) in THF (15 mL) over 30 min at −10 °C. The mixture was stirred at room temperature for 2 h. The reaction was quenched with water, which was extracted with dichloromethane. The combined organic layers were dried over anhydrous magnesium sulfate and then filtered. All the volatiles were removed under vacuum to give a crude product, which was purified by silica gel column chromatography with a mixture of petroleum ether and ethyl acetate (2/1, v/v) as the eluent to offer 6 as a white solid (650 mg, 85%). M.p.: 136−137 °C. 1H NMR (400 MHz, CDCl3) δ 8.07 (s, 1H), 7.71 (t, J = 8.0 Hz, 1H), 7.61−7.55 (m, 3H), 5.97 (s, 1H), 4.56 (s, 2H), 3.69 (s, 1H), 2.57 (s, 3H), 2.28 (s, 3H). 13C{1H} NMR (101 MHz, CDCl3) δ 151.5, 150.4, 149.1, 141.6, 140.9, 125.5, 124.0, 112.1, 109.6, 108.1, 55.7, 15.2, 13.6. HRMS (ESI) m/z calcd for C14H15N5O [M + Na]+ 292.1168, found 292.1181. (1-(6-(3,5-Dimethyl-1H-pyrazol-1-yl)pyridin-2-yl)-1H-pyrazol-3-yl)methanol (7). A similar procedure was used as the preparation of 6. White solid (1.5 g, 75%). M.p.: 141−142 °C. 1H NMR (400 MHz, CDCl3) δ 8.27 (d, J = 4.0 Hz, 1H), 7.76 (t, J = 8.0 Hz, 1H), 7.66 (d, J = 8.0 Hz, 1H), 7.62 (d, J = 8.0 Hz, 1H), 6.40 (d, J = 4.0 Hz, 1H), 5.99 (s, 1H), 4.71 (s, 2H), 2.66 (s, 3H), 2.28 (s, 3H). 13 C{1H} NMR (101 MHz, CDCl3) δ 155.4, 151.8, 150.3, 149.2, 141.4, 140.8, 128.0, 112.2, 109.6, 108.1, 106.6, 58.9, 15.2, 13.6. HRMS (ESI) m/z calcd for C14H15N5O [M + Na]+ 292.1168, found 292.1168. 2-(3,5-Dimethyl-1H-pyrazol-1-yl)-6-(4-(methoxymethyl)-1Hpyrazol-1-yl)pyridine (8). Under a N2 atmosphere, compound 6 (500 mg, 1.9 mmol) was added to a suspension of NaH (60% dispersion in mineral oil; 90 mg, 2.3 mmol) in THF (10 mL) at 0 °C. CH3I (475 mg, 3.3 mmol) was added and the solution was stirred at 0 °C for 30 min. The reaction was warmed to room temperature and stirred for 2 h. Water was added dropwise to quench the reaction, which was extracted with ethyl acetate. The combined organic layers were dried over anhydrous magnesium sulfate and then filtered. All the volatiles were removed under vacuum to give a crude product, which was purified by chromatography on silica gel with a mixture of petroleum ether and ethyl acetate (5:1, v/v) as the eluent to give 8 as a white solid (496 mg, 94%). M.p.: 81−82 °C. 1H NMR (400 MHz, CDCl3) δ 8.33 (s, 1H), 7.81 (t, J = 8.0 Hz, 1H), 7.73 (d, J = 4.0 Hz, 2H), 7.68 (s, 1H), 5.96 (s, 1H), 4.39 (s, 2H), 3.35 (s, 3H), 2.68 (s, 3H), 2.25 (s, 3H). 13C{1H} NMR (101 MHz, CDCl3) δ 151.9, 150.1, 149.3, 142.1, 141.3, 140.8, 126.2, 120.6, 112.3, 109.5, 108.1, 65.2, 57.8, 15.2, 13.6. HRMS (ESI) m/z calcd for C15H17N5O [M + Na]+ 306.1325, found 306.1304. 1-(6-(3,5-Dimethyl-1H-pyrazol-1-yl)pyridin-2-yl)-1,4-dihydroindeno[1,2-c]pyrazole (11). Under a N2 atmosphere, compound 1020(1.2 g, 6.4 mmol) was added to a solution of compound 921 (1.3 g, 6.4 mmol) in acetic acid (25 mL). The mixture was refluxed at 117 °C for 4 h. The reaction mixture was neutralized with saturated sodium bicarbonate aqueous solution, which was then extracted with ethyl acetate. The combined organic phases were dried over anhydrous magnesium sulfate and then filtered. All the volatiles were removed under vacuum to give a crude product, which was purified by silica gel column chromatography with a mixture of petroleum ether and ethyl acetate (6/1, v/v) as the eluent to obtain 11 as a yellow solid (2.0 g, 95%). M.p.: 161−162 °C. 1H NMR (400 MHz, CDCl3) δ 8.28 (d, J = 8.0 Hz, 1H), 8.03 (t, J = 8.0 Hz, 1H), 7.91 (d, J = 8.0 Hz, 1H), 7.75−7.72 (m, 2H), 7.55 (d, J = 8.0 Hz, 1H), 7.33−7.28 (m, 2H), 6.09 (s, 1H), 3.70 (s, 2H), 2.48 (s, 3H), 2.42 (s, 3H). 13C{1H} NMR (101 MHz, CDCl3) δ 151.8, 151.0, 150.5, 149.3, 149.0, 141.6, 141.0, 136.3, 132.7, 129.8, 126.7, 125.7,

of the OH group significantly affects the catalytic activity of complexes, and introducing a CH2OH group to the 4′-position of the pyrazolyl moiety is a suitable choice. The results disclosed here demonstrate that a simple ligand modification bestows dramatic changes to catalysis, which provides a general strategy for exploring bifunctional ligands to construct effective catalysts.

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EXPERIMENTAL SECTION

General Considerations. Unless otherwise noted, all the materials were purchased from commercial suppliers and used as received. Solvents were freshly distilled by standard procedures prior to use. All 1H and 13C{1H} NMR spectra were recorded on a Bruker 400 MHz spectrometer, and 31P{1H} NMR spectra were recorded on a JEOL JNM-ECZ600R NMR spectrometer. The NMR chemical shift values refer to CDCl3 (δ (1H), 7.26 ppm; δ (13C), 77.16 ppm), DMSO-d6 (δ (1H), 2.50 ppm; δ (13C), 39.52 ppm), CD3OD (δ (1H), 3.31 ppm; δ (13C), 49.00 ppm). High-resolution mass spectra (HRMS) were obtained on a micrOTOF-Q II mass spectrometer. Crystals were collected on a Bruker SMART APEX II CCD (Mo−Kα radiation, λ = 0.71073 Å) and a Bruker APEX III (Mo−Kα radiation, λ = 0.71073 Å). Elemental analyses (C, H, and N) were performed on a PerkinElmer 2400C elemental analyzer. 2-(3,5-Dimethyl-1H-pyrazol-1-yl)-6-fluoropyridine (3). Under a nitrogen atmosphere, NaH (60% dispersion in mineral oil; 9.0 g, 0.23 mol) was added to DMF (60 mL) in a 100 mL roundbottom Schlenk flask, followed by the addition of 2,6-difluoropyridine (20.0 g, 0.17 mol) via syringe. After the mixture was cooled to 0 °C, 3,5-dimethyl-1H-pyrazole (16.7 g, 0.17 mol) was added in small portions over 1 h. Once H2 evolution ceased, the reaction was warmed to room temperature and stirred for another 4 h. Water was added dropwise to quench the reaction, which was extracted with dichloromethane. The combined organic layers were dried over anhydrous magnesium sulfate and then filtered. All the volatiles were removed under vacuum to give a crude product, which was purified by silica gel column chromatography with a mixture of petroleum ether and ethyl acetate (10/1, v/v) as the eluent to afford 3 as a white solid (24.0 g, 73%). M.p.: 37−38 °C. 1H NMR (400 MHz, CDCl3) δ 7.85−7.79 (m, 1H), 7.75 (t, J = 4.0 Hz, 1H), 6.73−6.71 (m, 1H), 5.98 (s, 1H), 2.64 (s, 3H), 2.27 (s, 3H). 13C{1H} NMR (101 MHz, CDCl3) δ 161.1 (d, J = 232.2 Hz), 151.4 (d, J = 20.2 Hz), 149.7, 142.1 (d, J = 10.1 Hz), 141.4, 110.8, 109.3, 104.0 (d, J = 40.4 Hz,), 14.3, 13.0. HRMS (ESI) m/z calcd for C10H10FN3 [M + H]+ 192.0931, found 192.0931. Ethyl 1-(6-(3,5-Dimethyl-1H-pyrazol-1-yl)pyridin-2-yl)-1Hpyrazole-4-carboxylate (4). Under a N2 atmosphere, NaH (60% dispersion in mineral oil; 1.6 g, 38.8 mmol) was added to DMF (40 mL) in a round-bottom Schlenk flask. The solution was cooled to 0 °C, followed by the addition of ethyl 1H-pyrazole-4-carboxylate (5.4 g, 38.8 mmol). The mixture was stirred at room temperature for 30 min until the gas evolution ceased. 2-(3,5-Dimethyl-1H-pyrazol-1-yl)6-fluoropyridine (5.9 g, 30.5 mmol) was added and stirred at 110 °C for 6 h. Water was added dropwise to quench the reaction, which was extracted with dichloromethane. The combined organic layers were dried over anhydrous magnesium sulfate and then filtered. All the volatiles were removed under vacuum to give a crude product, which was purified by silica gel column chromatography with a mixture of petroleum ether and ethyl acetate (15/1, v/v) as the eluent to furnish the product 4 as a white solid (7.7 g, 81%). M.p.: 135−136 °C. 1H NMR (400 MHz, CDCl3) δ 8.80 (s, 1H), 8.06 (s, 1H), 7.89−7.81 (m, 2H), 7.77 (d, J = 4.0 Hz, 1H), 6.00 (s, 1H), 4.35−4.30 (m, 2H), 2.71 (s, 3H), 2.27 (s, 3H), 1.36 (t, J = 8.0 Hz, 3H). 13C{1H} NMR (101 MHz, CDCl3) δ 162.7, 152.2, 150.5, 148.7, 142.9, 141.5, 141.2, 130.0, 117.1, 113.5, 109.9, 108.8, 60.6, 15.3, 14.4, 13.7. HRMS (ESI) m/z calcd for C16H17N5O2 [M + Na]+ 334.1274, found 334.1274. Ethyl 1-(6-(3,5-Dimethyl-1H-pyrazol-1-yl)pyridin-2-yl)-1Hpyrazole-3-carboxylate (5). A similar procedure was employed as the preparation of 4. White solid (5.7 g, 88%). M.p.: 135.5−136.2 °C. 1 H NMR (400 MHz, CDCl3) δ 8.37 (d, J = 4.0 Hz, 1H), 7.84 (t, J = E

DOI: 10.1021/acs.organomet.8b00791 Organometallics XXXX, XXX, XXX−XXX

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Organometallics 123.4, 114.6, 112.0, 109.2, 28.9, 13.8, 13.8. HRMS (ESI) m/z calcd for C20H17N5 [M + H]+ 328.1556, found 328.1556. 1-(6-(3,5-Dimethyl-1H-pyrazol-1-yl)pyridin-2-yl)indeno[1,2c]pyrazol-4(1H)-one (12).22 tBuOOH (aqueous solution 70%; 13 mL, 110.0 mmol) was added dropwise to a solution of compound 11 (3.6 g, 11.0 mmol) and CrO3 (200 mg, 2.2 mmol) in dichloromethane (25 mL) at room temperature over 30 min. The mixture was refluxed for 12 h. The mixture was washed with water, dried over anhydrous magnesium sulfate, and then filtered. All the volatiles were removed under vacuum to give a crude product, which was purified by silica gel column chromatography with a mixture of petroleum ether and ethyl acetate (5/1, v/v) as the eluent to furnish 12 as a yellow solid (3.6 g, 95%). M.p.: 226−227 °C. 1H NMR (400 MHz, CDCl3) δ 8.04 (t, J = 8.0 Hz, 1H), 7.92 (d, J = 4.0 Hz, 1H), 7.87 (d, J = 4.0 Hz, 1H), 7.79−7.75 (m, 2H), 7.58 (t, J = 4.0 Hz, 1H), 7.29 (t, J = 4.0 Hz, 2H), 6.07 (s, 1H), 2.45 (s, 3H), 2.37 (s, 3H). 13C{1H} NMR (101 MHz, CDCl3) δ 183.8, 157.6, 151.6, 150.7, 149.5, 141.2, 141.1, 140.2, 136.3, 133.5, 133.2, 130.2, 125.4, 124.2, 123.9, 116.1, 112.6, 109.4, 13.6. HRMS (ESI) m/z calcd for C20H15N5O [M + Na] 364.1168, Found 364.1158. 1-(6-(3,5-Dimethyl-1H-pyrazol-1-yl)pyridin-2-yl)-1,4-dihydroindeno[1,2-c]pyrazol-4-ol (13). Compound 12 (3.0 g, 8.8 mmol) was dissolved in 100 mL of anhydrous methanol in a 250 mL round-bottom flask. After the mixture was cooled to 0 °C, sodium borohydride (1.0 g, 26.4 mmol) was added in several portions. The reaction mixture was stirred at room temperature for 2 h. The methanol was removed by rotary evaporator, and the residue was extracted with dichloromethane. The combined organic phase was dried over anhydrous magnesium sulfate, filtered, and all the volatiles were removed under vacuum to give a crude product, which was purified by silica gel column chromatography with a mixture of petroleum ether and ethyl acetate as the eluent (1/1, v/v) to provide 13 as a white solid (2.5 g, 83%). M.p.: 228−229 °C. 1H NMR (400 MHz, CDCl3) δ 8.03−7.95 (m, 2H), 7.83 (d, J = 8.0 Hz, 1H), 7.75 (s, 1H), 7.66 (d, J = 8.0 Hz, 1H), 7.60 (d, J = 8.0 Hz, 1H), 7.29 (d, J = 8.0 Hz, 1H), 7.21 (t, J = 8.0 Hz, 1H), 6.03 (s, 1H), 5.49 (s, 1H), 2.39 (s, 3H), 2.36 (s, 3H). 13C{1H} NMR (101 MHz, CDCl3) δ 152.4, 151.1, 150.3, 150.1, 148.0, 141.2, 140.9, 136.5, 133.2, 131.0, 128.4, 127.8, 125.4, 123.7, 114.9, 112.0, 109.0, 68.1, 13.6, 13.5. HRMS (ESI) m/z calcd for C20H17N5O [M + Na]+ 366.1325, found 366.1324. General Procedures for the Preparation of Complexes 14− 18. Under a N2 atmosphere, a mixture of RuCl2(PPh3)3 (710 mg, 0.74 mmol) and ligand 6 (200 mg, 0.74 mmol) in toluene (20 mL) was refluxed for 6 h. The mixture was cooled to ambient temperature to precipitate a red-brown microcrystalline solid. The solid was filtered off, washed with diethyl ether (3 × 15 mL), and dried under vacuum to afford the desired product. 14: red-brown crystalline solid (410 mg, 79%). M.p.: disintegrated at 295 °C. 1H NMR (400 MHz, DMSO-d6) δ 9.09 (s, 1H), 8.17 (s 1H), 7.94−7.87 (m, 2H), 7.49 (d, J = 8.0 Hz, 1H), 7.37−7.34 (m, 3H), 7.26−7.23 (m, 12H), 6.47 (s, 1H), 5.50 (t, J = 5.6 Hz, 1H), 4.43−4.33 (m, 2H), 2.70 (s, 3H), 2.49 (s, 3H). 13C{1H} NMR (101 MHz, DMSO-d6) δ 158.0, 150.0, 149.7, 146.2, 146.0, 139.1, 132.8, 132.6, 132.5, 130.6, 130.2, 129.8, 128.5, 128.4, 128.3, 113.1, 108.0, 107.0, 53.9, 14.4. 31P{1H} NMR (162 MHz, DMSO-d6) δ 32.3 (s, PPh3). HRMS (ESI) m/z calcd for C32H30Cl2N5OPRu [M − Cl]+ 668.0919, found 668.0915. Anal. Calcd for C32H30Cl2N5OPRu: C, 54.63; H, 4.30; N, 9.95. Found: C, 54.59; H, 4.27; N, 9.90. 15: red-brown crystalline solid (1.5 g, 69%). M.p.: disintegrated at 310 °C. 1H NMR (400 MHz, DMSO-d6) δ 9.17 (d, J = 4.0 Hz, 1H), 7.95 (t, J = 8.0 Hz, 1H), 7.88 (d, J = 8.0 Hz, 1H), 7.50 (d, J = 8.0 Hz, 1H), 7.39−7.36 (m, 3H), 7.25 (t, J = 8.0 Hz, 6H), 7.16−7.11 (m, 6H), 6.87 (d, J = 4.0 Hz, 1H), 6.47 (s, 1H), 5.64 (t, J = 5.2 Hz, 1H), 5.14 (dd, J1 = 5.4 Hz, J2 = 14.1 Hz, 1H), 4.35 (dd, J1 = 5.4 Hz, J2 = 14.1 Hz, 1H), 2.71 (s, 3H), 2.56 (s, 3H). 13C{1H} NMR (101 MHz, DMSO-d6) δ 163.5, 158.6, 149.9, 146.1, 139.1, 133.3, 132.6, 132.5, 130.9, 130.5, 130.1, 128.4, 128.3, 113.5, 111.3, 107.8, 107.0, 55.7, 14.4, 14.0. 31P{1H} NMR (241 MHz, DMSO-d6) δ 31.0 (s, PPh3). HRMS (ESI) m/z calcd for C32H30Cl2N5OPRu [M − Cl]+ 668.0919,

found 668.0912. Anal. Calcd for C32H30Cl2N5OPRu: C, 54.63; H, 4.30; N, 9.95. Found: C, 54.57; H, 4.28; N, 9.91. 16: yellow crystalline solid (585 mg, 92%). M.p.: disintegrated at 245 °C. 1H NMR (400 MHz, DMSO-d6) δ 9.39 (s, 1H), 8.24 (s, 1H), 8.04 (d, J = 8.0 Hz, 1H), 7.97−7.91 (m, 1H), 7.52 (d, J = 8.0 Hz, 1H), 7.36−7.33 (m, 3H), 7.27−7.24 (m, 11H), 7.19−7.17 (m, 1H), 6.49 (s 1H), 4.33 (s, 2H), 3.11 (s, 3H), 2.69 (s, 3H), 2.52 (s, 3H). 13 C{1H} NMR (101 MHz, DMSO-d6) δ 157.9, 149.9, 149.5, 146.4, 146.1, 138.9, 132.5, 132.4, 132.2, 130.5, 130.1, 128.3, 128.2, 128.1, 123.5, 113.0, 108.1, 107.2, 63.6, 57.0, 14.3. 31P{1H} NMR (241 MHz, DMSO-d6) δ 32.2 (s, PPh3). HRMS (ESI) m/z calcd for C33H32Cl2N5OPRu [M − Cl]+ 682.1076, found 682.1175. Anal. Calcd for C33H32Cl2N5OPRu: C, 55.23; H, 4.50; N, 9.76. Found: C, 55.27; H, 4.55; N, 9.70. 17: yellow crystalline solid (385.4 mg, 83%). M.p.: disintegrated at 315 °C. 1H NMR (400 MHz, DMSO-d6) δ 8.36 (s, 1H), 8.11−8.10 (d, J = 4.0 Hz, 1H), 8.03 (t, J = 8.0 Hz, 1H), 7.97 (d, J = 8.0 Hz, 1H), 7.72 (d, J = 8.0 Hz, 1H), 7.63 (d, J = 8.0 Hz, 1H), 7.56 (t, J = 8.0 Hz, 1H), 7.49 (t, J = 8.0 Hz, 1H), 7.34−7.27 (m, 9H), 7.23 (d, J = 8.0 Hz, 6H), 6.50 (s, 1H), 3.82 (d, J = 24 Hz, 1H), 3.62 (d, J = 24 Hz, 1H), 2.73 (s, 3H), 2.54 (s, 3H). 13C{1H} NMR (101 MHz, DMSO-d6) δ 157.8, 150.6, 150.3, 149.6, 149.3, 146.3, 141.9, 139.4, 132.7, 132.6, 131.9, 130.7, 130.3, 130.1, 129.6, 128.4, 128.3, 127.6, 126.9, 122.0, 113.2, 108.3, 107.5, 29.2, 14.4, 14.4. 31P{1H} NMR (241 MHz, DMSO-d6) δ 31.7 (s, PPh3). HRMS (ESI) m/z calcd for C38H32Cl2N5PRu [M − Cl]+ 726.1128, found 726.1126. Anal. Calcd for C38H32Cl2N5PRu: C, 59.92; H, 4.24; N, 9.20. Found: C, 59.89; H, 4.26; N, 9.15. 18: yellow crystalline solid (626 mg, 69%). M.p.: disintegrated at 308 °C. 1H NMR (400 MHz, CD3OD) δ 8.36 (s, 1H), 8.25 (s, 1H), 7.99 (t, J = 8.0 Hz, 3H), 7.89 (t, J = 8.0 Hz, 2H), 7.73−7.70 (m, 3H), 7.61 (d, J = 4.0 Hz, 1H), 7.57 (t, J = 8.0 Hz, 2H), 7.51−7.48 (m, 2H), 7.38−7.31 (m, 18H), 7.18 (s, 13H), 6.36 (d, J = 12 Hz, 2H), 5.54 (s 1H), 5.35 (s, 1H), 2.82 (s, 3H), 2.76 (s, 3H), 2.69 (s, 3H), 2.60 (s, 3H). 13C{1H} NMR (101 MHz, CD3OD) δ 158.3, 158.3, 158.3, 153.9, 153.7, 151.1, 151.0, 150.1, 149.9, 149.8, 146.8, 146.7, 142.6, 142.3, 139.9, 139.9, 137.0, 135.9, 133.1, 133.1, 131.1, 131.0, 130.8, 130.7, 130.6, 129.9, 129.7, 129.4, 129.4, 129.2, 129.0 128.8, 128.8, 128.7, 128.2, 128.1, 127.1, 127.0, 125.8, 122.7, 122.5, 113.7, 109.1, 109.0, 108.3, 108.0, 67.2, 66.9, 65.4, 15.7, 14.9, 14.7. 31P{1H} NMR (241 MHz, CD3OD) δ 31.8 (s, PPh3), 31.6 (s, PPh3). HRMS (ESI) m/z calcd for C38H32Cl2N5OPRu [M − Cl]+ 742.1077, found 742.1034. Anal. Calcd for C38H32Cl2N5OPRu: C, 58.69; H, 4.15; N, 9.01. Found: C, 58.75; H, 4.18; N, 8.92. X-ray Diffraction Studies. Single crystals of complexes 14−17 suitable for X-ray crystallographic determination were obtained by layering the corresponding solution in CHCl3/MeOH with hexane (1/3, v/v) at 25 °C. Single crystals of 18 suitable for X-ray crystallographic study were grown by diffusion of diethyl ether vapor into a solution in MeOH and CHCl3 at 25 °C. Single crystal X-ray diffraction studies for complexes 16 and 17 were carried out on a Bruker APEX II CCD diffractometer with graphite-monochromated Mo−Kα radiation (λ = 0.71073 Å), and the complexes 14, 15, and 18 were carried out on a Bruker APEX III diffractometer with graphitemonochromated Mo−Kα radiation (λ = 0.71073 Å). All of the structures were solved by direct methods and refined by full-matrix least-squares on F2 using the SHELX-97 program. All non-hydrogen atoms were refined with anisotropic thermal parameters, and all hydrogen atoms were included in calculated positions and refined with isotropic thermal parameters riding on those of the parent atoms. The disordered solvent molecules which could not be restrained properly were removed using the SQUEEZE route. The graphical representation of the molecular structures was carried out using Ortep32. CCDC numbers for complexes 14−18: 1874446−1874450. Typical Procedures for the Transfer Hydrogenation Reaction of Ketones. Under a N2 atmosphere, a reaction solution was prepared by dissolving ketone (2.0 mmol) and complex 14 (2.8 mg, 0.004 mmol) in 2-propanol (20 mL). The mixture was stirred at 82 °C for 10 min, and then 0.4 mL of 0.1 M iPrOK in 2-propanol (0.04 mmol) was introduced to initiate the reaction. At the stated time, 0.1 F

DOI: 10.1021/acs.organomet.8b00791 Organometallics XXXX, XXX, XXX−XXX

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Organometallics mL of the reaction mixture was sampled and immediately diluted with 0.5 mL of 2-propanol precooled at 0 °C, and filtered through a short pad of Celite to remove the complex catalyst to quench the reaction. The resultant filtrate was used for GC analysis.

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R. H. Ketone Asymmetric Hydrogenation Catalyzed by P-NH-P′ Pincer Iron Catalysts: An Experimental and Computational Study. ACS Catal. 2017, 7, 316−326. (6) Kuriyama, W.; Matsumoto, T.; Ogata, O.; Ino, Y.; Aoki, K.; Tanaka, S.; Ishida, K.; Kobayashi, T.; Sayo, N.; Saito, T. Catalytic Hydrogenation of Esters. Development of an Efficient Catalyst and Processes for Synthesising (R)-1,2-Propanediol and 2-(1-Menthoxy)ethanol. Org. Process Res. Dev. 2012, 16, 166−171. (7) (a) Spasyuk, D.; Smith, S.; Gusev, D. G. Replacing Phosphorus with Sulfur for the Efficient Hydrogenation of Esters. Angew. Chem., Int. Ed. 2013, 52, 2538−2542. (b) Spasyuk, D.; Vicent, C.; Gusev, D. G. Chemoselective Hydrogenation of Carbonyl Compounds and Acceptorless Dehydrogenative Coupling of Alcohols. J. Am. Chem. Soc. 2015, 137, 3743−3746. (8) Filonenko, G. A.; Aguila, M. J. B.; Schulpen, E. N.; van Putten, R.; Wiecko, J.; Müller, C.; Lefort, L.; Hensen, E. J. M.; Pidko, E. A. Bis-N-heterocyclic Carbene Aminopincer Ligands Enable High Activity in Ru-Catalyzed Ester Hydrogenation. J. Am. Chem. Soc. 2015, 137, 7620−7623. (9) (a) Alberico, E.; Sponholz, P.; Cordes, C.; Nielsen, M.; Drexler, H.-J.; Baumann, W.; Junge, H.; Beller, M. Selective Hydrogen Production from Methanol with a Defined Iron Pincer Catalyst under Mild Conditions. Angew. Chem., Int. Ed. 2013, 52, 14162−14166. (b) Andérez-Fernández, M.; Vogt, L. K.; Fischer, S.; Zhou, W.; Jiao, H.; Garbe, M.; Elangovan, S.; Junge, K.; Junge, H.; Ludwig, R.; Beller, M. A Stable Manganese Pincer Catalyst for the Selective Dehydrogenation of Methanol. Angew. Chem., Int. Ed. 2017, 56, 559−562. (10) Baratta, W.; Chelucci, G.; Gladiali, S.; Siega, K.; Toniutti, M.; Zanette, M.; Zangrando, E.; Rigo, P. Ruthenium(II) Terdentate CNN Complexes: Superlative Catalysts for the Hydrogen-Transfer Reduction of Ketones by Reversible Insertion of a Carbonyl Group into the Ru-H Bond. Angew. Chem., Int. Ed. 2005, 44, 6214−6219. (11) Chakraborty, S.; Dai, H.; Bhattacharya, P.; Fairweather, N. T.; Gibson, M. S.; Krause, J. A.; Guan, H. Iron-Based Catalysts for the Hydrogenation of Esters to Alcohols. J. Am. Chem. Soc. 2014, 136, 7869−7872. (12) (a) Käβ, M.; Friedrich, A.; Drees, M.; Schneider, S. Ruthenium Complexes with Cooperative PNP Ligands: Bifunctional Catalysts for the Dehydrogenation of Ammonia-Borane. Angew. Chem., Int. Ed. 2009, 48, 905−907. (b) Marziale, A. N.; Friedrich, A.; Klopsch, I.; Drees, M.; Celinski, V. R.; Schmedt auf der Günne, J.; Schneider, S. The Mechanism of Borane-Amine Dehydrocoupling with Bifunctional Ruthenium Catalysts. J. Am. Chem. Soc. 2013, 135, 13342−13355. (13) (a) Zeng, F.; Yu, Z. Exceptionally Efficient Unsymmetrical Ruthenium(II) NNN Complex Catalysts Bearing a Pyridyl-Based Pyrazolyl-Imidazolyl Ligand for Transfer Hydrogenation of Ketones. Organometallics 2008, 27, 2898−2901. (b) Zeng, F.; Yu, Z. Construction of Highly Active Ruthenium(II) NNN Complex Catalysts Bearing a Pyridyl-Supported Pyrazolyl-Imidazolyl Ligand for Transfer Hydrogenation of Ketones. Organometallics 2009, 28, 1855−1862. (14) Dubey, A.; Khaskin, E. Catalytic Ester Metathesis Reaction and Its Application to Transfer Hydrogenation of Esters. ACS Catal. 2016, 6, 3998−4002. (15) Ogata, O.; Nakayama, Y.; Nara, H.; Fujiwhara, M.; Kayaki, Y. Atmospheric Hydrogenation of Esters Catalyzed by PNP-Ruthenium Complexes with an N-Heterocyclic Carbene Ligand. Org. Lett. 2016, 18, 3894−3897. (16) (a) Zhang, J.; Leitus, G.; Ben-David, Y.; Milstein, D. Facile Conversion of Alcohols into Esters and Dihydrogen Catalyzed by New Ruthenium Complexes. J. Am. Chem. Soc. 2005, 127, 10840− 10841. (b) Zhang, J.; Leitus, G.; Ben-David, Y.; Milstein, D. Efficient Homogeneous Catalytic Hydrogenation of Esters to Alcohols. Angew. Chem., Int. Ed. 2006, 45, 1113−1115. (c) Balaraman, E.; Khaskin, E.; Leitus, G.; Milstein, D. Catalytic transformation of alcohols to carboxylic acid salts and H2 using water as the oxygen atom source. Nat. Chem. 2013, 5, 122−125. (d) Xie, Y.; Ben-David, Y.; Shimon, L. J. W.; Milstein, D. Highly Efficient Process for Production of Biofuel

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00791. The crystal structures of complexes 14−18. Selected bond lengths (Å) and angles (deg), and X-ray crystallographic structure parameters and refinement data for complexes 14−18. The copies of NMR spectra for new compounds, including 1H, 13C, and 31P (PDF) Accession Codes

CCDC 1874446−1874450 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Fanlong Zeng: 0000-0002-4293-5133 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the Science and Technology Department of Shaanxi Province (2018KW-032) and the Key Science and Technology Innovation Team of Shaanxi Province (2017KCT-37) for financial support.

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REFERENCES

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DOI: 10.1021/acs.organomet.8b00791 Organometallics XXXX, XXX, XXX−XXX